1. Field of the Invention
The present invention relates to a processing method using a pulsed laser beam, a processing apparatus using a pulsed laser beam which carries out this method, crystallization method using a pulsed laser beam, a laser crystallization apparatus which carries out this method, and a display device.
2. Description of the Related Art
A machining technology using an excimer laser which is a pulsed laser beam has been extensively studied and developed for process applications such ablation (evaporation), heating or melting. In recent years, an ablation technology has been extensively reviewed with respect to soft organic matters such as plastic, or ceramic, quartz and others which are called machining hard materials in particular. Since an excimer laser has a high photon energy (equal to or above 5 eV) and thereby directly acts on coupling between molecules, high machining performance is realized.
The excimer laser processing technology has been extensively used for, e.g., activation using an excimer laser beam intended to activate a semiconductor implanted layer, quartz ablation (evaporation) for forming a fiber grating, machining of a nozzle in an inkjet printer or the like.
In usual laser processing, continuous machining is carried out. Even if a light intensity of laser irradiation is not normal, machining is continued, and a method of again machining (repairing) that part is used later. For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-246484 discloses a processing method which again performs laser irradiation with respect to a non-normal machining point if a light intensity of laser beam irradiation is not normal.
Furthermore, in recent years, an excimer leaser annealing (in this case, laser processing is based on heating) technology which polycrystallizes an amorphous film in particular has been utilized, as a machining technology using a pulsed laser beam, in a technology of realizing high performance of a thin film transistor which is used in a drive element such as a liquid crystal display.
In order to realize high performance of the thin film transistor, a crystallized silicon thin film having a large grain diameter must be formed. It is known that, when a thin film transistor is formed in a crystal grain, a mobility of the transistor is improved ten times or more.
As a method of forming a crystal grain having a large grain diameter in an amorphous semiconductor thin film, there is a method described in “Journal of The Surface Science Society of Japan, Masakiyo Matsumura, Vol. 21, No. 5, pp. 278-287, 2000”.
In any application, microfabrication of micron order is required. If there is a failure of laser oscillation in one execution of laser irradiation due to any factor, there occurs a problem of a machining defect if nothing is fed back to a laser device. Specifically, when laser irradiation is not carried out or a light intensity of laser irradiation is lower than a set value, a region which is not machined well is generated, resulting in a reduction in a fair quality ratio (a yield ratio) of a final product.
Further, in crystallization based on laser annealing for other pulsed laser processing applications, if there is a part which has not been irradiated with a laser beam or a part in which an irradiation light intensity of a laser beam has not been enough, this part is not crystallized at all, or crystallization or activation becomes insufficient. Therefore, intended uniformity of characteristics or electrical characteristics cannot be obtained, which is a serious problem.
Furthermore, in a case where a laser processing method is used in a field using a laser processing method except the semiconductor field, e.g., machining of an ink discharge hole part of an inkjet head in an inkjet printer, if a laser beam is not applied to a given part or an irradiation light intensity of a laser beam is insufficient, a hole is not opened or hole machining becomes insufficient, and a yield ratio is likewise reduced in this field, resulting in a serious problem.
Usually, in the excimer laser, ultraviolet radiation has high machining properties and ultraviolet radiation demonstrates excellent absorption characteristics with respect to an amorphous semiconductor film in crystallization, and hence the excimer laser is suitable for crystallization. However, the excimer laser has instability in oscillation characteristics. As a result of actually executing a crystallization process by continuously and extensively applying a pulsed laser beam, the excimer laser device has the following drawbacks. Specifically, even though a trigger signal is externally input, no oscillation occurs, or a light intensity is extremely weak (e.g., not greater than ½ of a set value). In crystallization, even though a trigger signal for light emission control is input to the excimer laser device from the outside, a case of no oscillation is generated every 1000 pulses and the like. Alternatively, even though the same trigger signal is supplied, a light intensity is extremely weak, and it is not greater than a set value in some cases. That is because the excimer laser performs pulsed oscillation by a system based on high-voltage discharge in a gas. This is characteristics of a current element called “thyratron” used in the excimer laser, which is unavoidable at the present day. Although an excimer laser which does not use thyratron has been developed, a large load is imposed on a power supply circuit, thereby reducing reliability or increasing a cost.
This problem is realized as great irregularities of a machining conformation in a machining process, which is interference of mass production.
A “light-unemitting shot” described herein means a shot by which machining becomes incomplete, and includes both a “case where laser light is not generated at all” and a “case where a light intensity is low or a light emission time is short”.
As a method of judging whether a shot of a leaser beam is a light-unemitting shot, it is general to adopt a method which places an optical component which partially divaricates light for laser beam intensity monitoring between a laser beam source and a processing target material in order to monitor divergent light such as described in Patent Reference 1. If it is determined that an intensity of the monitored divergent light is not normal, processing which returns to a non-normal processing point and again applies a laser beam can be considered. In the processing method in the above-described reference, however, there are the following two problems. That is, firstly, since a position at which a laser beam intensity is monitored is not a processing target material surface, there is a problem that an accurate laser beam intensity on the processing target material surface cannot be grasped. Secondly, moving an X-Y stage to return to an abnormal processing point and again applying a laser beam has no problem in case of rough processing of several-ten μm, but there is a limit in a mechanical accuracy in microfabrication which requires a positional accuracy of a processing point which is approximately 0.1 to 1.0 μm, and hence there is a problem that uniform processing is impossible (this problem can be solved by improving the mechanical accuracy).
In view of the above-described problems, it is an object of the present invention to provide a laser processing method using a pulsed laser beam which again allows a light emitting operation on the scene even if a light-unemitting shot occurs in application of a pulsed laser beam by a laser beam source so that accurate machining is possible without a defect, and a laser processing apparatus using a pulsed laser beam.
Next, in case of forming a liquid crystal drive circuit which forms a large display screen of a liquid crystal display device in a crystallized region, there are the following problems. In a process of sequentially moving a pulsed laser beam to a predetermined irradiation position in a large display screen to thereafter apply the pulsed laser beam, when any problem exists in laser irradiation of one given shot (an abnormal shot), a serious problem occurs in crystallization of the irradiation position. For example, when laser irradiation is not performed or when a light intensity of laser irradiation is low, crystallization does not occur, or crystallization becomes insufficient. Therefore, when a transistor is formed in this region, transistor characteristics are considerably deteriorated, and a fair quality ratio (a yield ratio) of a final product is thereby lowered.
Further, besides the laser crystallization process, in activation of a semiconductor implanted layer based on laser annealing, when a laser beam is not applied or when an irradiation light intensity of a laser beam is low, this part is not activated at all, or activation becomes insufficient. Therefore, intended electrical characteristics cannot be obtained, which is a serious problem in quality management.
In view of the above-described problems, it is an object of the present invention to provide a crystallization method which detects an abnormal shot and again performs shooting to thereby improve a reduction in a yield ratio of crystallization even if the abnormal shot occurs in irradiation of a laser beam for crystallization, a laser crystallization apparatus and a display device.